As oil well drilling becomes increasingly complex, the importance of collecting and analyzing downhole data while drilling increases. Well logging instruments are often used to probe subsurface formations to determine formation characteristics.
Sonic tools are an example of well logging tools that may be used to provide information regarding subsurface acoustic properties that can be used to analyze the formation. This information may include the compressional wave speed, shear wave speed, borehole modes, and formation slowness. The information obtained by acoustic measurements has a number of applications, including, but not limited to, seismic correlation, petrophysics, rock mechanics and other areas.
During a typical sonic logging of a formation, an acoustic logging instrument or tool is lowered into a borehole that transverses the formation of interest. The acoustic logging tool may be mounted to the drill collar or other devices and directed downhole. Conventional acoustic logging tools include acoustic transducer elements such as a piezoelectric element. Generally, the acoustic transducer can convert electric energy to acoustic energy as well as acoustic energy to electric energy and may act as an acoustic source or an acoustic sensor. The acoustic logging tool typically includes a transmitter which performs as an acoustic source and emits acoustic energy into the formation and one or more receivers or acoustic sensors that receive acoustic energy. Once the acoustic logging tool is lowered into the formation, the transmitter may be periodically actuated to emit pulses of acoustic energy into the borehole. The emitted acoustic waves propagate through the borehole wall producing a reflection that is then detected by the receiver(s) which produce an electric signal in response. Attributes of the acoustic energy that is detected at the receiver(s) may then be used to characterize subsurface properties of the formation of interest.
However, the receiver(s) of the acoustic logging tool are typically also sensitive to undesired acoustic noise that may result from normal drilling operations. For instance, the undesired acoustic noise may propagate with reduced attenuation through the hard steel drill collar. The acoustic noise may then couple to the receiver of the acoustic logging tool and be converted into electrical noise along with the desired signal. This background noise may be a result of the downhole operations or produced by other acoustic sources and therefore, may introduce an error in the measurements by the acoustic logging tool. It is therefore desirable to provide an acoustic logging tool that can minimize or eliminate the susceptibility of the acoustic logging tool to the background noise.